Epoxy resins have been widely used as various dielectric and structural materials in casting, impregnating, laminating and molding techniques. In these fields, increasingly severe requirements are imposed on such materials. Heat resistance is one of key features desired of such materials.
Prior art well-known heat resistant epoxy resins include a variety of polyepoxidized resins, for example, epoxidized phenol novolak resins (e.g, Epikote 154 commercially available from Yuka Shell Epoxy K.K.), epoxidized cresol novolak resins (e.g, EOCN commercially available from Nihon Kayaku K.K.), methylenedianilinetetraepoxide, and epoxidized tri and tetra(hydroxyphenyl)alkanes.
The heat resistance of these polyepoxidized resins are relatively high, but still unsatisfactory in practice. They require long-term heating at elevated temperatures before they gain an acceptable strength, and are less satisfactory in processability.